In these years, the number of radio frequency interferences caused by information equipment is sharply increasing at home and overseas with the progress of advanced information.
There are many such cases including that police and government office radiocommunication frequencies are interrupted, and TV radio frequencies are interrupted by personal computers.
To coincide with the progress of electronic equipment whose operation is easily malfunctioned or made abnormal due to such electromagnetic wave interferences, the control of electromagnetic waves (EMI) is a worldwide issue.
The control is made by FCC (Federal Communication Commission) in the U.S.A. and by FTZ (Fernmelde Technisches Zentralant) which is the technical organization of the Ministry of Posts and Telecommunications in Germany.
Internationally, IEC (International Electrotechnical Commission) and its subordinate organization, CISPR (Comite International Special des Perturbations Radio Electriques), control the limited values and measuring methods of electromagnetic wave interferences caused by various electrical appliances and the standards of measuring equipment, and give recommendations to the member nations.
The control of electromagnetic wave interferences in Japan is voluntarily in effect by VCCI (Voluntary Control Council for Interference by data processing equipment and electronic office machines) since 1986.
In the electromagnetic wave (EMI) radiation test of electronic equipment, the measurement frequency is specified to be 30 MHz to 1,000 MHz according to each standard of CISPR (Comite International Special des Perturbations Radio Electriques), FCC (Federal Communication Commission), and VDE.
Consequently, a radio wave absorber is used to absorb incident radiowave energy and to convert into heat energy.
Since a minimum frequency of 30 MHz has a very long wavelength of 10 m, it is difficult to obtain a high absorbing property at a low frequency band of 100 MHz or below.
For example, a carbon-impregnated urethan absorber is required to have a length of 5 m or more to obtain the absorption of 20 dB or more at a frequency band of 30 MHz or higher.
Thus, when the urethan absorber is used to dispose a radio shielding room, the wave absorber is often insufficient in absorbing capacity to provide the radio shielding room with a sufficient low-frequency characteristic.
In these years, an excellent ferrite wave absorber is being used, and its performance and miniaturization have been improved steeply, enabling to conform to ANSI C63.4 using the ferrite wave absorber alone.
For the ferrite wave absorber, a ferrite tile of 10 cm.times.10 cm is generally used. It has a disadvantage that the absorbing capacity at a low-frequency band of 100 MHz or below is degraded because of small gaps formed between the ferrite tiles when they are tiled.
In the case of a pyramid type wave absorber in combination with ferrite, a large pyramid type wave absorber having a height of 0.9 m to 2.7 m is required to ensure the wave absorbing capacity at a low frequency band of 30 MHz to 100 MHz, and particularly at 100 MHz or below.
Therefore, the large pyramid type wave absorber is required to be made of light-weight materials, and in most cases has heretofore used a support material such as urethane foam (sponge-like), expanded polystyrene or rubber, which is impregnated or mixed with carbon graphite. And, it is generally used in the form of a plate, a mountain or a pyramid to provide for a wide frequency band.
A plate type wave absorber (FIG. 10) has a flat face into which a radiowave enters, and is generally used as a single layer wave absorber. It is to be understood that a two-layer wave absorber or multi-layer wave absorber using two layers or more is basically designed in the form of a plate.
In FIG. 10, reference numeral 31 stands for a single-layer or multi-layer plate type wave absorbing material, 32 for a ferrite tile disposed on the back face of the wave absorber 31, and 33 for a metallic reflector disposed on the back face of the ferrite tile 32.
An angle type wave absorber (FIG. 11) has its front face made in the form of triangle mountains made of the wave absorbing material. This form has advantages that making an angle front face linearly increases gradually a wave attenuation constant on that face, so that a wide-band characteristic can be obtained.
In FIG. 11, reference 41 stands for a hollow angle type wave absorbing material, 42 for a ferrite tile disposed on the back face of the wave absorber 41, and 43 for a metallic reflector disposed on the back face of the ferrite tile 42.
A pyramid type wave absorber (FIG. 12) scatters an incident wave in various directions. Therefore, it is difficult to know in which direction the reflected wave is directed. Most of the imported wave absorbers are pyramid type wave absorbers.
In FIG. 12, reference 51 stands for a hollow pyramid type wave absorbing material made of urethane foam, 52 for a ferrite tile disposed on the back face of the wave absorber 51, and 53 for a metallic reflector disposed on the back face of the ferrite tile 52.
However, the above materials have a disadvantage that they are very flammable.
Therefore, nonflammable materials have been eagerly demanded, and they are now more eagerly demanded with the increasing needs for them.
In the U.S.A., a restriction has been imposed on incombustibility, and products having a flame retarder mixed into the above urethane material have been announced but still have various disadvantages. Thus, satisfactory products have not been produced yet.
Nonflammable materials have been produced with antimony chloride or the like mixed as a flame retarder, but have disadvantages that they are deteriorated soon, deformed and inferior in durability.
On the other hand, wave absorbers using a cement-based material such as a gas concrete or calcium silicate plate as a nonflammable material have been tried, but not commercialized because they are too heavy to be used and hard to produce as the wave absorbers (e.g., Japanese Patent Application Laid-open Prints No. 62-42498, No. 64-44097, No. 2-27798, No. 4-294599, etc.).
A wave absorber which is produced with carbon graphite impregnated has disadvantages that the impregnated graphite content is varied, its production is not controled easily, and this wave absorber is hardly made uniform in quality (e.g., Japanese Patent Application Laid-open Print No. 62-45100).